Aphid Alert 2003, No. 6, July 25

Potato insect update for the Northern Great Plains, week ending July 25

Aphid flight activity across the Northern Great Plains increased considerably during the past week. The most abundantly represented species were bird cherry-oat aphid, buckthorn aphid, and turnip aphid. The trap data did not show increases in green peach aphid activity, but colonization of potato by this species is now occurring. Growers are advised to scout their fields, paying particular attention to field margins adjoining fallow, e.g., headlands or planting skips.

This summer participating Minnesota and North Dakota potato growers have entered 30 potato fields in a large-scale University of Minnesota demonstration/research project. The project's objective is to evaluate possible benefits of targeting the year's first insecticide [Monitor (methamidophos)] spray treatment for green peach aphid along field margins. The first 6 fields were sprayed today. Twenty four more are scheduled to be sprayed next week. We have previously shown that initial colonization of potato fields by green peach aphids occurs almost exclusively along field margins that are kept fallow. For the first 10-14 days following initial invasion of a potato field by winged green peach aphid, more than 90% of colonizing green peach aphids are found within the first 10-20 yards from a fallow border. This colonizing behavior of green peach aphid provides growers the opportunity to target insecticide applications on those portions of the field where treatment will be most advantageous. In preliminary studies, potato growers were able to reduce their use of Monitor by 80% by treating only crop margins. The advantages of using targeted insecticide applications include effective aphid control at less cost, conservation of natural enemies in the untreated center of the field, and reduced selection for insecticide resistance. Monitor resistant green peach aphids have been reported from some fields in the Pacific Northwest.

Minnesota-North Dakota aphid data, week ending July 22

table showing Minnesota and North Dakota aphid data for the week ending July 22, 2003

Manitoba aphid data, week ending July 24

table showing Manitoba aphid data for the week ending July 24, 2003

Additional information on the aphid situation in Manitoba can also be found at www.gov.mb.ca/agriculture/crops/insects/index.html, and the Manitoba Agriculture and Food potato hotline at 1-800-428-6866.

Late Blight Situation

During the past week, weather conditions in the Northern Great Plains were generally not conducive to development of Late Blight. There is a chance of severe thunderstorms late this afternoon in eastern North Dakota and northwestern Minnesota and unstable weather is expected to continue into the weekend. In some locations rainfall may be heavy. The Late Blight Disease Severity Threshold has been reached over much of the region. If conditions become favorable and inoculum is high, Late Blight could develop within a week. So far, the only known case of Late Blight this summer occurred near Carberry, Manitoba.

Potato late blight status reports

Mineral oils for PVY control

Minnesota seed potato inspectors have reported current season spread of mosaic (PVY) in some seed potato fields. Since PVY is transmitted in a non-persistent manner by many aphid vectors, including a number of aphid species that do not colonize potato, insecticides tend to be considerably less effective in reducing PVY incidence than in reducing spread of potato leafroll (PLRV). Mineral oils may be an effective alternative. A product locally available is Aphoil (Agsco, Grand Forks).

Research has shown that application of nontoxic crop oils can substantially reduce spread of PVY and other non-persistently transmitted plant viruses vectored by aphids. The exact mode of which mineral oils interfere with virus transmission is unknown, but it appears to be a complex contact-based action. To be effective, complete coverage of the leaf surface with oil is essential and the oil needs to be reapplied frequently (e.g., weekly) to cover new growth. The universal inhibitory effect of mineral oils on non-persistent virus transmission supports the hypothesis that oils must interfere with some basic underlining process of the virus-aphid mouthpart interaction. However, mineral oils also appear to interfere with normal leaf exploratory processes of aphids and increase pre-probing time on oil coated leaves.

Mineral oils are a mixture of aromatic, naphthenic, and paraffinic structures. Paraffinic oils are the most effective at inhibiting non-persistent virus transmission. Optimal qualities of a mineral crop oil are: viscosity-gravity constant 0.790-0.819, viscosity 66-150 SUS, boiling range 370-420 ºC, paraffin-pourpoint below 0 ºC, and mean molecular weight 340-380. The unsulfonated residue content of a mineral crop oil should be nearly free of phytotoxic aromatic structures.

To interrupt transmission of non-persistently-transmitted viruses, it is important to achieve thorough coverage and even distribution of oil particles on the leaf surface. The label rate for application of Aphoil is a 2% v/v solution applied in at least 20 gal of water per acre. Applications should be made at high pressure. Addition of an emulsifier and use of spray nozzles that produce a fine droplet size (e.g., TeeJet® TX-4) is recommended. Mineral oils should not be applied with fungicides containing tin or copper.

Reduction of PVY spread using a mineral crop oil (Aphoil) and two commonly used potato aphicides. Crookston, 2000.

Experimental plots were planted on 24 May, 2000 with Red LaSoda minitubers. Individual plots were 4 rows x 25 plants (12 x 25 ft). Plots were arranged in a completely randomized block design with each treatment replicated four times. Two rows of potatoes, cv. Red Pontiac infected with PVY were planted as a border surrounding the experimental area. Aphoil applications were made using a tractor-mounted boom sprayer at 100 gal/A and 40 psi on 14, 21, 30 July, 4, 9, 15 August (total of 6 applications). Fulfill and Monitor applications were made on 30 July and 9 August.

Guide to the Field Identification of Wingless Aphids on Potato

(click here for a PDF of the Guide)

photo of a potato aphid photo of a buckthorn aphid photo of a foxglove aphid photo of a green peach aphid photo of a cotton aphid
Potato Aphid, Macrosiphum euphorbiae (Thomas) Buckthorn Aphid, Aphis nasturii (Kaltenbach) Foxglove Aphid, Aulacorthum solani (Kaltenbach) Green Peach Aphid, Myzus persicae (Sulzer) Cotton (=melon) aphid, Aphis gossypii (Kaltenbach)

Subscriber Alert

This is the sixth issue of Aphid Alert for the 2003 growing season. This newsletter is intended to alert seed potato producers in the Northern Great Plains to flight activity by aphid species known to be potential vectors of potato viruses. We report results weekly on the WWW, by e-mail to subscribers, and by surface mail to all Minnesota and North Dakota seed potato growers. The hard copy and e-mail versions of Aphid Alert report aphid capture data available as of the date they are mailed. The WWW version will be updated as additional data becomes available. To become an e-mail subscriber to Aphid Alert 2003, send us an e-mail message with the word "subscribe" in subject line. Note that current subscribers need not resubscribe. If you have no interest in receiving this newsletter by e-mail, please reply with the word "unsubscribe" in the subject line.